1.Field of the Invention
This invention relates generally to a method for measuring electrical properties of a semiconductor wafer and, more particularly, for high speed measuring of threshold voltage and average surface doping.
2. Description of Related Art
Conventional methods of characterizing the quality of a dielectric formed on a silicon surface of a semiconductor wafer include capacitance-voltage (CV) techniques. Measurements of threshold voltage and average surface doping concentration are taken in this manner. Specifically, CV measurements require that a measurement site on and adjacent the silicon surface of the semiconductor wafer be first biased into deep depletion, a non-equilibrium condition. The measurement site must then be returned to thermodynamic equilibrium before threshold voltage and average surface doping can be measured. Equilibrium is reached by exposing the measurement site to a light source after it reaches deep depletion.
With reference to FIG. 1, heretofore, a CV curve 8 was generated by superimposing an AC signal on a DC signal and changing the DC signal applied to the measurement site from a starting voltage having a first polarity to an ending voltage having a second, opposite polarity and measuring the resultant capacitance. When the starting voltage is applied, an accumulation region is observed of approximately constant capacitance 10. As the voltage approaches zero and changes polarity, the capacitance decreases as the measurement site becomes biased into depletion 12 and this capacitance eventually reaches deep depletion 14. At deep depletion 14, a light is directed at the measurement site while the voltage is held constant 16 whereupon the measurement site returns to equilibrium 18. When the measurement site reaches equilibrium 18, the light is turned off and the voltage is decreased 20 toward zero. The capacitance levels off during this stage. From the thus derived CV curve, threshold voltage and average surface doping can be determined in manners known in the art.
This process provides a slow means for reaching equilibrium, taking approximately xc2xd minute for measurement. However, for production speed, threshold voltage and average surface doping must be measured very quickly, on the order of seconds. Therefore, a need exists for a method for measuring threshold voltage and average surface doping quickly.
It is, therefore, an object of this invention to provide an improved method for reaching equilibrium whereupon threshold voltage and average surface doping concentration can be calculated quicker (i.e., on the order of seconds), as compared to the conventional methods.
A method according to the present invention includes exposing a measurement site to a high intensity light immediately before a measurement sweep begins. A CV measurement sweep is then made with the voltage increasing from zero volts to its maximum positive or negative voltage. (A maximum positive voltage is used for p-type silicon whereas a maximum negative voltage is used for n-type silicon.) When the voltage reaches its maximum voltage, the light is turned off and the response of the measurement site to the absence of light thereon is measured until the capacitance reaches equilibrium.
The method allows thermodynamic equilibrium to be reached very quickly after the CV measurement sweep and, thereby, for very high speed threshold voltage and average surface doping concentration measurements.